The present invention discloses an asphalt composition suitable for use in air blowing and the process of air blowing the same, both using hydrohalic acids, particularly hydrobromic acid, as catalysts.
Air-blowing of asphalt is a process by which stock asphalt is converted to an asphalt product having more desirable properties by the forced introduction of air by blowing at temperatures ranging from 300.degree. F. to 600.degree. F. In its unprocessed forms, asphalt may be unsuitable for particular applications due to excessive hardness or a too low softening point. The degree of hardness is referred to as penetration. The modification of the softening point/penetration properties by air blowing permits the manufacture of the aspahlt to specifications otherwise not possible. These products are called "Industrial or Roofing Asphalts". Desirable properties for Industrial Asphalts are given in ASTM D312-78. Additionally, it is well known that the use of a catalyst during air blowing further improves the softening point/penetration relationship of the asphalt. Catalyst use also reduces the time required to bring the asphalt to the desired softening point, a significant economic benefit.
Commonly used and well-known air-blowing catalysts, many of which have been patented, include ferric chloride, FeCl.sub.3, U.S. Pat. No. 1,783,186, phosphorous pentoxide, P.sub.2 O.sub.5, U.S. Pat. No. 2,450,756, aluminum chloride, AlCl.sub.3, U.S. Pat. No. 2,200,914, boric acid, U.S. Pat. No. 2,375,117, copper sulfate CuSO.sub.4, zinc chloride ZnCl.sub.2, phosphorous sesquesulfide, P.sub.4 S.sub.3, phosphorous pentasulfide, P.sub.2 S.sub.5 and phytic acid, C.sub.6 H.sub.6 O.sub.6 (H.sub.2 PO.sub.3).sub.6. Also useful as catalysts are phosphoric acid H.sub.3 PO.sub.4 and ferrous chloride FeCl.sub.2, U.S. Pat. No. 4,338,137, sulfonic acid U.S. Ser. No. 445,400, now U.S. Pat. No. 4,440,579, and alkali metal hydroxides, U.S. Ser. No. 537,892 now abandoned. By far, the most useful and commonly used of the catalysts are ferric chloride and phosphoric acid.
The primary reason for the popularity of FeCl.sub.3 and H.sub.3 PO.sub.4 is the fact that they are readily obtained and relatively inexpensive to use. They do, however, have serious drawbacks. In particular, the use of ferric chloride as a catalyst contributes to two negative characteristics of the asphalt which raise problems in meeting desired specifications. One characteristic, called skinning, results from the heating of industrial asphalts in the presence of air at elevated temperatures. A tough, insoluble skin is formed on the surface of the asphalt which is extremely insoluble in the asphalt itself. This skin causes problems both to the refiner and the asphalt customer. It has been found that the skinning tendency of the asphalt increases when ferric chloride is used as the catalyst in air blowing.
A second undesirable characteristic resulting from the use of ferric chloride catalysts is known as "fallback". Fallback is a drop in the softening point which air-blown asphalt may undergo when held at a stable elevated temperature in an oxygen-depleted atmosphere. When asphalt is held over time at elevated temperatures, the softening point/penetration ratio reduces or "falls back" outside of the desired specification range even though it is being held at a constant temperature. This may be caused or aggravated by various mixture components, particularly FeCl.sub.3.